= Solution
The assumed central <black hole> mass is $M_\bullet\sim10^8M_\odot\simeq2\times10^{38}\,\mathrm{kg}$. A mass alone does not fix radiated energy: specify a formation mechanism and its <radiative efficiency of black-hole accretion>. For radiatively efficient accretion, if a fraction $\epsilon$ of the supplied rest-mass energy escapes and the remainder increases the <black hole> mass, then $M_\bullet=(1-\epsilon)M_{\rm supplied}$ and
$$
\boxed{E_\bullet=\frac{\epsilon}{1-\epsilon}M_\bullet c^2,qquad
\langle L_\bullet\rangle=\frac{\epsilon}{1-\epsilon}\frac{M_\bullet c^2}{t_{\rm dyn}}}.
$$
Taking the customary order-of-magnitude efficiency $\epsilon\sim0.1$ gives
$$
E_\bullet\simeq2\times10^{54}\,\mathrm J,qquad
\langle L_\bullet\rangle\simeq10^{39}\,\mathrm W\simeq3\times10^{12}L_\odot.
$$
\b[The coeval accretion phase would be a quasar-scale source, hundreds of times brighter than a typical present $L_*$ galaxy.] Compared directly with the preceding gravitational-collapse estimate,
$$
\frac{E_\bullet}{E_{\rm rad}}\simeq\frac23\frac{\epsilon}{1-\epsilon}
\frac{M_\bullet}{M}\left(\frac c\sigma\right)^2\sim10^2.
$$
Despite its small mass fraction, <black-hole accretion> extracts a fraction of rest-mass energy, while galactic collapse releases only an energy scale $M\sigma^2\ll Mc^2$. The numerical <luminosity> is conditional: a <radiatively inefficient accretion flow>, direct collapse or assembly by mergers need not radiate this amount over the galactic clock, and the question supplies no unique efficiency.
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